In electrostatic charge image developing toners used in devices such as digital copying machines and laser printers, surface-treated silica fine powders have hitherto been used as external additives for toners for the purpose of improving the fluidity of the toners or stabilizing the charging property of the toners. These silica fine powders are required to have high hydrophobicity for the purpose of reducing the charge amount variation due to humidity and to be low in aggregation and high in dispersion for the purpose of uniformly covering the surface of the toners. With respect to the specific surface area of the silica fine powders, ultrafine powders having a specific surface area of approximately 200 to 500 m2/g are used; however, it has been verified that when the image formation is repeated, the silica ultrafine powders are embedded in the surface of the toner particles, thus for example, the fluidity, the triboelectric charge amount and the transferability of the toner are degraded, and image failure is caused.
There is a method in which an inorganic fine powder having specific surface area of less than 80 m2/g and accordingly a relatively large particle size is used in combination in order to reduce the embedment of the silica ultrafine powder (Patent Literature 1, Patent Literature 2 and Patent Literature 3). An inorganic fine powder having a relatively large particle size develops a spacer effect to reduce the stress caused by the mutual direct contact of the toner particles. Accordingly, for example, a method for achieving an elongation of the operating life of a toner by suppressing the embedment of the silica ultrafine powder has been adopted.
However, an inorganic fine powder having a relatively large particle size tends to be smaller in charge amount as compared with an ultrafine powder, and causes the decrease of the charge amount when the external addition amount of such an inorganic fine powder is increased for the purpose of improving the spacer effect. Moreover, recently, because of the acceleration of the low-temperature fixing of the toner, when the toner is stored over a long term, there has been a growing problem of the degradation of the storage stability of the toner due to the mutual blocking of the toner particles. Accordingly, the storage stability and the chargeability of the fine powder having a relatively large particle size have been demanded to be further improved.